It is proposed to develop and use a quantitative method for the separation of cells according to the surface density of specific antigenic determinants of cells. Antibodies specific for these sites will be chemically coupled to particles in the submicron size range (radius of equivalent sphere from 50 to 500 nm) and suitably coated. The primary coat is obtained by suspension (redox) co-polymerization of two (or more) acrylamide (or other) derivatives. Antibody-carrying particles will be reacted with live cells in the non-capping condition and the conjugates will be fractionated by either one of two modes. To the extent these particles have a significantly higher density than the cells, gravitational separation will be performed in a counter-flow velocity gradient. And to the extent these particles are magnetizable, separation will be done in a constant magnetic field gradient. A 10 kG/cm gradient constant to within a better than 1% in a volume of 0.2 mm x 2 mm x 3 mm will be used in a constant (but continuously adjustable) counter flow to measure the response characteristics of particles and cells with particles attached. The data would be used for the design of a larger apparatus with counter flow gradient. The magnetic version is particularly suitable for cells with low surface densities of antigenic determinants. In principle, the counter flow gradient versions allow relatively large quantities of cells to be separated into any number of well-defined fractions at one time. Initially, cells with phosphorylcholine receptors will be fractionated. Other fractionations will be developed with collaborators who specialize in specific cell lines. Quantitative fractionations of cells (and cell-fragments as well) in preparative quantities are quite important in several fields of biomedical research. The proposed method would allow such quantitative fractionations.